1. Technical Field
The present invention relates to a gas laser oscillator having a function to determine the initiation of an electric discharge.
2. Description of the Related Art
In a gas laser oscillator, high-frequency power is applied to a discharge tube to excite a laser gas contained in the discharge tube. If a high voltage is applied to the discharge tube when an electric discharge is not yet initiated, an impedance mismatch may occur between the laser power supply and the discharge tube, as a result of which an excessively large current may flow in the laser power supply and an excessively large voltage may be applied to the discharge tube. If this happens, the laser power supply or the discharge tube may be damaged, or to prevent such damage, the gas laser oscillator is forcefully turned off.
A gas laser oscillator is known, which is provided with an auxiliary electrode for producing an auxiliary electric discharge separately from a main electric discharge for generating a laser. This is intended to reduce a temporary voltage increase at the time of initiating the electric discharge by producing an auxiliary electric discharge prior to the main electric discharge for providing laser output.
JP-A-2011-222586 discloses a gas laser oscillator including a determining unit for determining initiation of an electric discharge. The determining unit is configured to determine initiation of an electric discharge by comparing a rate of changes in the voltage of the discharge tube in response to an output command from the power supply with data collected when an electric discharge is normally produced in the discharge tube.
The method disclosed in JP-A-2011-222586 includes providing a command voltage increasing in a stepwise manner, and monitoring the voltage of the discharge tube in response to the command voltage, in order to determine whether an electric discharge is initiated or not. However, in the case of an auxiliary electric discharge with a minuscule output level, it has been difficult to determine initiation of the auxiliary electric discharge, since changes in the voltage of the discharge tube are very small.
FIG. 5 is a graph depicting the relationship between the command voltage CV′ supplied to a laser power supply unit and the voltage V′ applied to the discharge tube. “CV′1” indicates the value of the command voltage CV′ when the auxiliary electric discharge is initiated, and “CV′2” indicates the value of the command voltage CV′ when the main electric discharge is initiated. As can be seen from the graph of FIG. 5, the rate of changes in the applied voltage V′ with respect to the command voltage CV′ changes significantly before and after the main electric discharge is initiated. Accordingly, it is possible to determine the initiation of the main electric discharge by continuously monitoring the rate of changes in the applied voltage V′. On the other hand, the rate of changes in the applied voltage V′ does not substantially change before and after the auxiliary electric discharge is initiated. Therefore, it is difficult to accurately determine the time at which the auxiliary electric discharge is initiated.
As can be seen from FIG. 5, the auxiliary electric discharge is initiated at the command voltage CV′1 which is smaller than the command voltage CV′2 required to initiate the main electric discharge. Therefore, instead of determining at the time at which the auxiliary electric discharge is initiated, it is possible to at least determine whether or not the auxiliary electric discharge has already been initiated, by determining the initiation of the main electric discharge.
For example, in the related art disclosed in JP-A-2011-222586, the command voltage to the power supply unit needs to be switched so that the command voltage increases in a stepwise manner, in order to determine the initiation of the main electric discharge. However, a voltage value of the discharge tube cannot be detected stably immediately after the switching of the command voltage, due to a time delay associated with a time constant of the monitoring circuit or with the timing of A/D conversion, or depending on the relationship with a control cycle of CNC (computer numerical control) equipment. As a result, a waiting time of sufficient length, for example, at least 200 ms, has to be provided. If the process for determining the discharge initiation is performed before the waiting time elapses, the determination of the discharge initiation may be incorrect, since the voltage being applied to the discharge tube cannot be monitored accurately.
FIG. 6 is a graph for explaining a method for determining the initiation of an electric discharge in the gas laser oscillator according to the above related art. In the graph of FIG. 6, a solid line indicates the command voltage CV′ supplied to the power supply unit, and each square represents the voltage applied to the discharge tube detected in a predetermined sampling cycle. The command voltage CV′ is controlled in accordance with a ramp command so that the command voltage CV′ gradually increases over the period from time t1 to time t2 (for example, over a period of several hundred milliseconds) until it reaches V′1. At time t2, the command voltage CV′ is switched to a step command to start the discharge initiation determining process, but there is a waiting time from time t2 to time t3, in order to avoid the effects of the above-mentioned time delay or the like. The length of the waiting time varies, depending on the hardware configuration, but is generally not shorter than 200 ms, and this waiting time has been a factor causing a delay in the discharge initiation determining process. In FIG. 6, a white square indicates a sampled value taken when the applied voltage V′ cannot be detected stably, i.e., a sampled value taken during the waiting time.
Therefore, there is a need for a gas laser oscillator including a determining unit that can reliably determine initiation of an electric discharge in a short period of time.